White light emitting device

ABSTRACT

A white light emitting device includes a structure for emitting white light having at least four wavelengths by using two or less LEDs, where the LEDs include a blue/green LED emitting blue and green wavelengths of light. The device also includes means for emitting red wavelength of light.

CLAIM OF PRIORITY

This application claims the benefit of Korean Patent Application No.2005-130652 filed on Dec. 27, 2005, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a white light emitting device, and moreparticularly, to a white light emitting device using up to two LEDs toachieve superior color reproducibility and high efficiency.

2. Description of the Related Art

In general, a white LED apparatus is preferred over a conventionalsmall-sized lamp or a fluorescent lamp as a backlight of a LiquidCrystal Display (LCD). The white LED apparatus can be manufactured byforming a ceramic phosphor layer on a light exiting surface of a blueLED.

The most representative type of conventional white LED apparatus isrealized by combining a GaN-based blue LED with YAG-based yellowphosphor. The blue light emitted from the blue LED excites the phosphorto emit yellow light. A mixture of blue light and yellow light isperceived as white light by the observer. Such a white LED apparatus hasadvantages like relatively high efficiency and low costs but alsodrawbacks like mediocre color reproducibility.

FIG. 1 shows a light emission spectrum of a conventional white lightemitting device manufactured by combining a GaN-based blue LED withYAG-based yellow phosphor. As shown in FIG. 1, the light intensity ishigh in the blue and yellow regions but low in the green and redregions. Therefore, such a white light emitting device does not wellexpress the green and red regions, thus having a mediocre colorrendering index and color reproducibility.

Alternatively, a white light LED apparatus may be realized by combiningblue, green and red LEDs. FIG. 2 shows light emission spectrums of sucha type of conventional white LED apparatuses composed of blue, green andred LEDs. In particular, FIG. 2 shows light emission spectrums of whiteLED apparatuses with different correlated color temperatures. Such whiteLED apparatuses have relatively good color reproducibility but with highpower consumption and low efficiency. Moreover, these white LEDapparatuses are not cost-effective with complex circuit configurations.

As a further alternative, LEDs can be used to realize a white lightemitting device by combining an ultraviolet LED (UV LED) withgreen/blue/red phosphor or a mixture of blue, green and red phosphors.Such a white light emitting device has superior color reproducibilitywith low costs, but is not yet commonly used and has low efficiency.Therefore, there exists a need for developing a white light emittingdevice which satisfies a need for high efficiency and high colorreproducibility.

SUMMARY OF THE INVENTION

The present invention has been made to solve the foregoing problems ofthe prior art and therefore an object of certain embodiments of thepresent invention is to provide a high-quality white light emittingdevice having high efficiency and superior color reproducibility.

According to an aspect of the invention for realizing the object, thereis provided a white light emitting device including: a structure foremitting white light having at least four wavelengths by using two orless light emitting diodes (LEDs), wherein the LEDs comprise ablue/green LED emitting blue and green wavelengths of light; and thedevice comprising means for emitting red wavelength of light.

The blue/green LED may be an InGaN-based LED. Also the red lightemitting means may be an InGaAlP-based LED. Alternatively, the red lightemitting means may be red phosphor or a red Photon Recycling System(PRS).

Preferably, the white light emitting device emits white light having alight emission spectrum with a blue peak wavelength in the range of 440to 470 nm, and a green peak wavelength in the range of 500 to 530 nm, ayellow peak wavelength in the range of 560 to 580 nm and a red peakwavelength in the range of 620 to 640 nm. More preferably, the whitelight emitting device emits white light having a light emission spectrumwith a blue peak wavelength in the range of 450 to 460 nm, a green peakwavelength in the range of 505 to 515 nm, and a yellow peak wavelengthin the range of 565 to 575 nm, and a red peak wavelength in the range of630 to 640 nm. In this case, the white light has a color rendering indexof at least 95.

According to an embodiment of the present invention, the white lightemitting device may further comprise yellow phosphor, wherein the redlight emitting means may comprise at least one selected from a groupconsisting of a red LED, a red PRS and red phosphor. In this case, thewhite light emitting device attains white light by mixing the blue,green, yellow and red colors. Preferably, the yellow phosphor has a peakwavelength in the range of 540 to 590 nm.

According to another embodiment of the present invention, the red lightemitting means may be an amber/red LED emitting amber and redwavelengths of light. In this case, the white light emitting deviceattains white light by mixing the blue, green, amber and red colors.

According to further another embodiment of the present invention, theblue/green LED is adapted to emit amber wavelength of light along withblue and green wavelengths of light, and the red light emitting meansmay be a red LED emitting red wavelength of light. In this case, thewhite light emitting device uses two LEDs to attain white light bymixing the blue, green, amber and red colors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing a light emission spectrum of a conventionalwhite light emitting device according to the prior art;

FIG. 2 is a graph showing light emission spectrums of another type ofconventional light emitting devices according to the prior art;

FIG. 3 is a schematic sectional view illustrating a white light emittingdevice according to an embodiment of the present invention;

FIG. 4 is a graph showing a light emission spectrum of the white lightemitting device shown in FIG. 3;

FIG. 5 is a graph showing external quantum efficiency of an InGaN-basedLED and an InGaAlP-based LED with respect to wavelength;

FIG. 6 is a schematic sectional view illustrating a white light emittingdevice according to another embodiment of the present invention;

FIG. 7 is a schematic sectional view illustrating a white light emittingdevice according to further another embodiment of the present invention;

FIG. 8 is a graph showing a light emission spectrum of the white lightemitting device shown in FIG. 7;

FIG. 9 is a schematic sectional view illustrating a white light emittingdevice according to still another embodiment of the present invention;

FIG. 10 is a graph showing a light emission spectrum of the white lightemitting device shown in FIG. 9;

FIG. 11 is a schematic sectional view illustrating a white lightemitting device according to yet another embodiment of the presentinvention; and

FIG. 12 is a graph showing a light emission spectrum of the white lightemitting device shown in FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will now be described indetail with reference to the accompanying drawings. The invention mayhowever be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the shapes and dimensions may beexaggerated for clarity, and the same reference numerals are usedthroughout to designate the same or like parts.

FIG. 3 is a schematic sectional view illustrating a white light emittingdevice according to an embodiment of the present invention. Referring toFIG. 3, the white light emitting device 100 includes a blue/green LED101 and a red LED 102 mounted in a cup-shaped reflecting recess 110 of apackage and yellow phosphor 103. The blue/green LED 101 is a single chipemitting both blue wavelength of light B and green wavelength of lightG. The yellow phosphor 103 absorbs the light generated from the LED 101to generate yellow light Y. The red LED 102 generates red wavelength oflight R. Preferably, the blue/green LED 101 is an InGaN-based LED andthe red LED 102 is an InGaAlP-based LED.

The blue/green LED 101 and the red LED 102 are encapsulated by alight-transmitting resin 105. The yellow phosphor 103 powders aredispersed in the resin 105. The yellow phosphor 103 may be for exampleYAG-based yellow phosphor which absorbs the blue light to generateyellow light. In this case, the YAG-based yellow phosphor 103 absorbs aportion of the output light of the blue/green LED 101 and generates theyellow light Y. Preferably, the yellow phosphor 103 emits light having apeak wavelength in the range of 540 to 590 nm.

The white light emitting device 100 with the above describedconfiguration outputs white light in a wide range of wavelength usingthe blue, green, yellow and red light beams generated from the LEDs 101and 102 and the yellow phosphor 103. Particularly, the white lightemitting device 100 generates visible rays with 4 wavelengths using onlytwo LEDs, thus improved in color reproducibility and efficiency at thesame time.

To realize white light having a color rendering index of at least 90, itis preferable that the white light emitting device 100 emits white lighthaving a light emission spectrum with a blue peak wavelength in therange of 440 to 470 nm, a green peak wavelength in the range of 500 to530 nm, a yellow peak wavelength in the range of 560 to 580 nm, and ared peak wavelength in the range of 620 to 640 nm. More preferably, theblue peak wavelength may range from 450 to 460 nm, the green peakwavelength may range from 505 to 515 nm, the yellow peak wavelength mayrange from 565 to 575 nm, and the red peak wavelength may range from 630to 640 nm. This allows realizing a high-quality white light emittingdevice having a color rendering index of at least 95.

With each of the peak wavelengths in each of the above preferablewavelength ranges, the white light emitting device 100 achieves asuperior color rendering index (or color reproducibility) and improvedpower and light efficiency. In particular, in the case where theblue/green LED 101 is an InGaN-based LED and the red LED 102 is anInGaAlP-based LED, the blue/green LED 101 may have a short peakwavelength of up to 470 nm and the red LED 102 may have a long peakwavelength of at least 620 nm to achieve further improved efficiency,which is confirmed as shown in FIG. 5.

FIG. 5 is a graph showing external quantum efficiency of an InGaN-basedLED and an InGaAlP-based LED with respect to the wavelength. As shown inFIG. 5, the InGaN-based LED exhibits higher external quantum efficiencywith a shorter wavelength (peak wavelength), and the InGaAlP-based LEDexhibits higher external quantum efficiency with a longer wavelength(peak wavelength). Using YAG-based phosphor generating a yellow regionof light together with an InGaN-based blue/green LED having a peakwavelength of up to 470 nm and an InGaAlP-based red LED having a peakwavelength of at least 620 nm, the white light emitting device canachieve high efficiency in an entire visible ray-region and highluminance of at least 50 lm/W.

FIG. 4 is a graph showing a light emission spectrum of the white lightemitting device shown in FIG. 3. As shown in FIG. 4, the white lightemitting device 100 emits white light having peak wavelengths in theblue, green, yellow and red regions at 455 nm, 510 nm, 570 nm and 635nm, respectively. Such a wide visible ray spectrum with 4 wavelengthsallows high color reproducibility and a high color rendering index,thereby resulting in a high-quality white light close to natural light.

FIG. 6 illustrates a white light emitting device according to anotherembodiment of the present invention. As shown in FIG. 6, the white lightemitting device 200 according to this embodiment is distinguished fromthe previously described white light emitting device 100 in that itincludes a red Photon Recycling System (PRS) 112 instead of the red LED.

In general, the PRS, like any phosphor, absorbs light from other lightsource to generate a different wavelength of light without any voltageapplied. However, unlike the phosphor, the PRS is made of semiconductormaterial. Such red PRS 112 is disposed on a light exiting surface of theblue/green LED 101, absorbing the blue light (or green light) emittedfrom the LED 101 to generate red light R. Using only one LED (blue/greenLED 101) to output white light having a wide spectrum with 4wavelengths, the white light emitting device 200 has high efficiency andsuperior color reproducibility.

FIG. 7 illustrates a white light emitting device 300 according tofurther another embodiment of the present invention. The white lightemitting device 300 according to this embodiment is distinguished fromthe previously described white light emitting device 100 in that itincludes red phosphor 107 instead of the red LED.

As shown in FIG. 7, the blue/green LED 101, the yellow phosphor 103 andthe red phosphor 107 emanate blue, green, yellow and red colors,respectively, thereby attaining a visible ray spectrum with 4wavelengths. Similar to the aforedescribed embodiment, only one LED(blue/green LED 101) is used in the white light emitting device 300 inthis embodiment to output white light with 4 wavelengths, achieving highefficiency and superior color reproducibility. For the red phosphor,phosphor having a composition, Sr_(x)Ca_(l-x)S:Eu²⁺, where x is greaterthan or equal to 0 and less than or equal to 1, can be used. Thephosphor, Sr_(x)Ca_(1-x)S:Eu²⁺ absorbs blue light to emanate red light.

FIG. 8 shows a light emission spectrum of the white light emittingdevice 300 shown in FIG. 7. As shown in FIG. 8, the light emissionspectrum exhibits peak wavelengths in the blue, green, yellow and redregions at 450 nm, 493 nm, 570 nm and 615 nm, respectively. With therespective peak wavelengths in the 4 wavelength regions, the white lightemitting device 300 achieves high color reproducibility and a high colorrendering index.

FIG. 9 illustrates a white light emitting device according to yetanother embodiment of the present invention. Referring to FIG. 9, thewhite light emitting device 400 includes a blue/green LED 101 mounted ina cup-shaped reflecting recess of a package and an amber/red LED 150.Therefore, in this embodiment, two LEDs (the blue/green LED and theamber/red LED) emit a blue light beam B, a green light beam G, an amberlight beam A and a red light beam R. Using only two LEDs to output whitelight having a wide spectrum with 4 wavelengths, the white lightemitting device 400 has high efficiency and superior colorreproducibility.

FIG. 10 shows a light emission spectrum of the white light emittingdevice 400 shown in FIG. 9. As shown in FIG. 10, the white lightemitting device 400 emits white light having a light emission spectrumwith peak wavelengths at 458 nm, 525 nm, 590 nm and 640 nm,respectively. With the respective peak wavelengths in a wide visible rayregion with 4 wavelengths, the white light emitting device 400 achieveshigh color reproducibility and a high color rendering index.

FIG. 11 illustrates a white light emitting device according to stillanother embodiment of the present invention. Referring to FIG. 11, thewhite light emitting device 500 includes a blue/green LED 170 mounted ina cup-shaped reflecting recess of a package and a red LED 102. Unlike inthe aforedescribed embodiments, the blue/green LED 170 emits not onlyblue and green light but also amber light. That is, the blue/green LED170 is a three-wavelength LED emitting blue, green and amber wavelengthsof light. Such a three-wavelength LED can be realized by altering the Incontent of an InGaN layer in an active layer of multiple quantum well.

The blue light B, green light G and amber light A emitted from theblue/green LED 170 and the red light R emitted from the red LED 102 aremixed to produce high-quality white light having a wide spectrum with 4wavelengths. The light emission spectrum of the white light emittingdevice 500 is shown in FIG. 12. As shown in FIG. 12, the white lightemitting device 500 has peak wavelengths at 450 nm, 500 nm, 570 nm and635 nm, respectively to output high-quality white light with 4wavelengths.

According to the present invention set forth above, the white lightemitting device uses two or less LEDs to output white light having awide spectrum with 4 wavelengths, thereby achieving superior colorreproducibility and high efficiency.

While the present invention has been shown and described in connectionwith the preferred embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. A white light emitting device comprising: a structure for emittingwhite light having at least four wavelengths by using two or less lightemitting diodes (LEDs), wherein the LEDs comprise a blue/green LEDemitting blue and green wavelengths of light; and the device comprisingred light emitting means.
 2. The white light emitting device accordingto claim 1, wherein the blue/green LED comprises an InGaN-based LED. 3.The white light emitting device according to claim 1, wherein the redlight emitting means comprises an InGaAlP-based LED.
 4. The white lightemitting device according to claim 1, wherein the red light emittingmeans comprises red phosphor.
 5. The white light emitting deviceaccording to claim 1, wherein the red light emitting means comprises ared Photon Recycling System (PRS).
 6. The white light emitting deviceaccording to claim 1, the white light has a light emission spectrum witha blue peak wavelength in the range of 440 to 470 nm, and a green peakwavelength in the range of 500 to 530 nm, a yellow peak wavelength inthe range of 560 to 580 nm and a red peak wavelength in the range of 620to 640 nm.
 7. The white light emitting device according to claim 1, thewhite light has a light emission spectrum with a blue peak wavelength inthe range of 450 to 460 nm, a green peak wavelength in the range of 505to 515 nm, and a yellow peak wavelength in the range of 565 to 575 nm,and a red peak wavelength in the range of 630 to 640 nm.
 8. The whitelight emitting device according to claim 7, the white light has a colorrendering index of at least
 95. 9. The white light emitting deviceaccording to claim 1, further comprising yellow phosphor, wherein thered light emitting means comprises at least one selected from a groupconsisting of a red LED, a red PRS and red phosphor.
 10. The white lightemitting device according to claim 9, wherein the yellow phosphor has apeak wavelength in the range of 540 to 590 nm.
 11. The white lightemitting device according to claim 1, wherein the red light emittingmeans comprises an amber/red LED emitting amber and red wavelengths oflight.
 12. The white light emitting device according to claim 1, whereinthe blue/green LED is adapted to emit amber wavelength of light alongwith blue and green wavelengths of light, and the red light emittingmeans comprises a red LED emitting red wavelength of light.